U can do sit ups if u had a vaginal delivery but mind u exercise isn't the only thing you need...1. Breastfeed ur baby exclusively for 6 months. 2. Drink plenty of water, eat lots of fruits and veggies, as much as u have to eat well to produce enough milk for ur baby, don't eat plenty food at once...u can eat little by little portion rather than eating a lot at a go! Then, give it time, u see results!